A typical high speed serial data communications link operates by combining a clock signal and a data signal into one signal stream. Generally, a transmitter and a receiver are located at opposite ends of the serial data communications link. The receiver recovers the clock signal, and then uses the recovered clock signal to sample the data signal. The data signal comprises a series of ones and zeros, indicated by high and low voltage levels. The data signal transitions from high to low, or low to high, to convey information. The transitions should occur away from the point in time when the recovered clock indicates that the level of the data signal is to be sampled.
A condition referred to as jitter occurs when the transitions in the data signal fail to occur at a preferred time relative to the recovered clock signal. Noise, poor signal propagation conditions, and imperfect transmitter and receiver design are common causes of jitter. Jitter generally manifests as a variation in the location of the transitions in the data signal relative to the clock signal. If the jitter is sufficiently severe, the data signal may transition at a point in time when it is due to be sampled, or even later. In such cases, the received data bit will be in error.
The period of the recovered clock defines the data rate. The period for transmission of one bit is called a unit interval (UI). By convention, the time period of a unit interval is normalized to 0.0 at the beginning of each interval and to 1.0 at the end of a unit interval. The data signal typically transitions from logic high to logic low or from logic low to logic high at the beginning and/or end of each unit interval. The data value is typically sampled in the middle of the unit interval, at a time corresponding to 0.5 UI, to avoid sampling the data signal during a data transition. Some receivers are designed to always sample in the middle of the unit interval. Other receivers are designed to allow sampling at an arbitrary location (phase) within the unit interval.
FIG. 1 is a graphical illustration 100 showing the effect of jitter on a serial data communications link. The diagram 100 includes a recovered clock signal 102 and an ideal data signal 104. In this example, the preferred alignment time is illustrated at 114 on an exemplary negative transitioning clock edge 110 of a clock pulse 111. The ideal data signal 104 includes an exemplary data transitioning edge 117 on a pulse 116. The unit interval 112 of the pulse 116 is shown from 0.0 through 1.0. As shown in FIG. 1, the alignment time 114 and the ideal data signal 104 transitioning at 0.5UI with respect to the alignment time 114 results in the nominal sample location at 0.5 UI, illustrated at 115.
A data signal having minimal jitter is illustrated at 106. The data signal 106 is offset in time from the ideal data signal 104, and from the clock signal 102, by an amount shown at 118. The data signal 106 includes an exemplary pulse 124. The jitter indicated at 118 is sufficiently small so that the pulse 124 is sampled at a point in time relative to the transitioning edge 110 of the clock signal 102 so that the sampled data represented by the pulse 124 will likely be accurate.
A data signal having significant jitter is illustrated at 108. The data signal 108 includes a pulse 126. In this example, a transitioning edge 127 of the pulse 126 occurs coincident to the nominal sample location 115. In this example, the negative transitioning edge 127 of the pulse 126 occurs coincident to the nominal sample time 115, so that when the data associated with the pulse 126 is sampled at the nominal sample time 115, the sampled value will be potentially in error.
It is desirable in serial data communications link design to ensure that the transmitter, channel, and receiver are all designed to keep jitter within acceptable bounds. An acceptable bound for jitter should also include a margin to allow for natural variation in hardware characteristics from device to device or over time and environmental changes for a single device. The Bit Error Ratio, BER, is representative of the ratio of erroneous bits received to the total number of bits transmitted during a defined period of time of operation of the serial data communications link. A BER measurement is performed by transmitting a known data pattern and performing a comparison of expected to actual data received at the receiver. Some transmitters and/or receivers include the ability to adjust various parameters, such as the phase of the receiver during sampling. A common way of depicting the relation of BER to a parameter such as receiver phase is by the use of a so called “eye pattern” or “eye width.” An eye pattern is a graphical way of visually indicating the performance of a communications link.
One previous method for measuring the eye width was to measure the BER at each phase setting for an equal amount of time. For receivers with a phase adjustment with many positions, this can be a long and error prone process. For example, for a receiver with 32 positions, running a bit error ratio test (BERT) for 1 minute at each position means that there will be 32 manually collected data points, and take over 32 minutes to collect the data.
The measurement of link quality generally includes running a series of BER measurements while sweeping the receiver's sampling phase across the unit interval. The transmitter sends the selected bit pattern continuously. Each measurement is performed by (1) setting the receiver's sampling phase offset, (2) clearing error and bit counters in the receiver, (3) running the receiver and associated error and bit counters for a period of time (the measurement “dwell” time), (4) unloading and saving the values in the error and bit counters after the desired dwell time, and (5) repeating this process for each phase offset. The result is a table of BER values versus receiver phase setting.
A common representation of the swept BER measurement is referred to as a “bathtub” plot. This plot gets its name from the distinctive curves, resembling the sloping ends of a bathtub. A disadvantage of a bathtub curve is that while mathematically accurate, careful examination and familiarity are required to develop a feel for the overall quality of the serial communications link. While acceptable for experts, novices find such a curve difficult to interpret.
Therefore, it is desirable to have the ability to quickly measure the eye width of a serial communications channel, and to have the ability to display the performance parameters of a serial communications link in an easy to read and intuitive manner.